An electric vehicle may include a vehicle chassis frame, a battery pack, and an electric motor. In some electric vehicles the total battery voltage for driving the vehicle is fairly high, e.g., 100-200 V or more. The high voltage circuits need to be isolated from the vehicle chassis frame for a variety of safety reasons, including protecting human health of users and technicians.
There are various international safety standards for electric vehicles. Among these international safety standards are European safety standards for electrified vehicles that require that the high voltage circuits are isolated from the chassis frame. An on-board system is required in some of the safety standards in order to detect frame faults. A frame fault is a fault in which the electrical isolation between the high voltage electrical system and the frame decreases below a threshold value. In the case of an electrically powered vehicle, examples of frame faults include the development of leakage paths from the battery pack, degradation of wires in an electric motor, or other faults in the high voltage electrical system.
The safety standards for electric vehicles reduce the risk of technicians or operators from being shocked. In particular, the safety standards provide protection for the situation that a user, who is grounded to the chassis, touches a high voltage terminal or an ungrounded part of the electrical system when there is an insulation failure. As long as the resistance between the chassis and the high voltage system is large enough (i.e., above a threshold level depending on the maximum voltage and other factors), the current that results when a person touches a high voltage element will be limited to a safe level, i.e., a level not hazardous to human health.
There are several known techniques to determine isolation resistance in an electric vehicle, but each of these has significant drawbacks. One way to determine isolation resistance is to measure the current into and out of the power source (e.g., at the battery leads), but an isolation monitor based on this approach that would be precise enough to meet safety standards would be prohibitively expensive for many applications. Another way to determine isolation resistance is to use a fixed impedance circuit to monitor frame voltage or frame leakage current. For example, a simple voltage divider to the chassis with equal value resistor could be used to detect short circuits to the positive or negative battery terminal. However, the inventors of the present application have recognized that this approach does not detect certain types of faults. In particular, the inventors have recognized that in an electric vehicle the battery packs have numerous batteries arranged in a series configuration such that some types of faults can occur at the effective “mid-point” of the battery pack or to a star connection of an induction motor, because the effect of the fault is symmetric. For example, a fault at the precise midpoint of a battery pack has a symmetric effect on voltages such that this type of midpoint fault is undetectable using a simple voltage divider.
Thus, while there are techniques in the prior art to measure isolation resistance, these techniques are generally prohibitively expensive for many applications and generally also have problems in detecting certain types of faults, such as mid-point faults.
Therefore what is desired is an improved apparatus, system, method, and computer program product to monitor electrical isolation.